2-Alkyl-5-hydroxypyrimidines, particularly the 2-t-butyl homolog, are advantageously employed as intermediates for the preparation of insecticidal O-5-pyrimidinyl phosphorothioates as described, for example, in U.S. Pat. Nos. 4,127,652 and 4,729,987. Typically, 2-alkyl-5-hydroxypyrimidines are prepared from the corresponding 2-alkylpyrimidine by selective halogenation of the 5-position followed by hydrolysis to the 5-hydroxy analog. ##STR1## Although fraught with difficulties, 2-alkyl-5-hydroxypyrimidines can be successfully prepared by this conventional approach; see, for example, U.S. Pat. No. 4,486,590 concerning chlorination and U.S. Pat. No. 4,379,930 concerning hydrolysis. Nevertheless, this scheme is not as straightforward as it appears to be.
Since most 2-alkylpyrimidines are not commercially available, the starting materials for this route must themselves be prepared. This is usually accomplished by reacting an alkanecarboxylic acid (RCO.sub.2 H) with 1,3-diaminopropane in the vapor phase; see, for example, U.S. Pat. No. 4,999,427. This reaction first involves a condensation/cyclization to an intermediate 2-alkyl-1,4,5,6-tetrahydropyrimidine followed by dehydrogenation to the 2-alkylpyrimidine. Thus, for example, 2-t-butyl-5-hydroxypyrimidine would typically be prepared from pivalic acid and 1,3-diaminopropane by the following scheme. ##STR2## It would be highly desirable to have a more direct route to 2-alkyl-5-hydroxypyrimidines that would entail fewer steps and fewer unit operations.
One approach to abbreviating the conventional scheme would be to employ an acyclic raw material already functionalized with oxygen which, when cyclized, would incorporate the oxygen functionality in the proper ring position. In accord with this approach, U.S. Pat. No. 4,880,929 discloses the cyclization of pivalic acid and 1,3-diamino-2-propanol to 2-t-butyl-5-hydroxy-1,4,5,6-tetrahydropyrimidine. However, this patent remains surprisingly silent concerning the subsequent dehydrogenation of this intermediate to the corresponding pyrimidinol.